This invention relates generally to reinforced plastic bodies and more particularly to posttensioned cement and concrete slabs.
Prestressing of reinforced concrete is known to increase its resistance to shearing and tensile stresses. Conventionally, concrete has been prestressed by either of two methods: pretensioning and posttensioning.
In pretensioning, the prestressing force is applied by means of high-strength steel wires or tendons which are arranged end to end between two fixed anchorages. The tendons are stretched to a high state of stress and the molds are filled with fresh concrete. After the concrete has hardened, the tendons are released from the anchorages and the stress in the tendons is transferred to the concrete, thereby placing the concrete under compression. However, difficulties in applying the tensile stresses and in regulating the magnitude of the forces which are ultimately imposed upon the hardened body has prevented pretensioning from being a completely satisfactory technique of prestressing concrete.
By contrast, in posttensioning the concrete is allowed to harden before the tendons are stressed. In order for the tendon to be stressed, however, it is necessary that the tendon be free to move in the axial direction with respect to the hardened concrete at the time of tensioning.
In one posttensioning method, therefore, the tendons are coated with grease or some other petroleum product to prevent them from bonding to the concrete. Thus, after the concrete hardens, the tendons can still move relative to the concrete when mechanically tensioned to place the concrete in compression. However, this method has been unsatisfactory for several reasons. The void formed between the tendon and the concrete by the grease provides a pathway for transmission of corrosive materials which may attack the tendon. Also, since the tendon is not bonded along its length, any failure will cause a total loss of prestressing effect of the tendon.
In another posttension method, conduit consisting of metal or rubber sleeves is placed in the forms and the concrete is cast around them. After the concrete hardens, the tendons are threaded through the sleeves and tensioned using special jacks anchored against the ends of the concrete member, which itself forms the abutment. After the concrete has hardened and the tendon is tensioned, the conduit is filled with grout. While this method overcomes some of the difficulties associated with pretensioning methods, it has not been completely satisfactory. Sealing the conduit with grout has been particularly inconvenient. In addition, any voids in or failure of the grout provides a pathway for transmission of corrosive materials which may attack the tendon.
In yet another posttensioning method, it has been proposed to coat the tendon with a low melting point or thermoplastic material. After the concrete is cured, the tendon is heated to a temperature that causes the coating to soften and the tendon to thermally expand in the axial direction. After the tendon is heated to the desired temperature, the ends of the tendon are secured to the concrete mass and the contraction of the tendon due to cooling places the concrete in compression. See, e.g, U.S. Pat. No. 2,414,011 of Karl P. Billner issued on Jan. 7, 1947, in which it is also said that the coating may be omitted where the body to be prestressed comprises a thermoplastic material. While this method overcomes the problems associated with the formation of voids in a grout, the amount of tensioning that may be achieved by this method is limited by the maximum temperature that the tendon may be subjected to without adversely affecting the concrete or the reinforcing characteristics of the tendon. Accordingly, this method has not won wide acceptance; with ordinary working materials, the temperature required to achieve the necessary thermal expansion of the tendon adversely affects the tendon and the concrete.
Consequently, a need has remained for a method for prestressing plastic bodies that will permit a degree of tensionsing greater than that which can be achieved through thermal expansion of the tendon alone, that will be resistant to corrosive attack and that will be convenient to use.